Fixed L-Pads

[Earl K]

I'm going to try and resolve the question of how good a mic the RS meter is, also. If it's good, then his driver isn't generating VHF, or his horn is killing it.

- Well, I couldn't wait. I just did a rough FR check on one of my RS SPL meters / it's approx @ 6 db down at 10K .

- With such a dramatic HF rolloff, here's no telling what MarkT(s)' network/le85 is really giving him .

- Also, I've edited post #50 ( regarding the network build ) .

[Zilch]

And 6 dB up at 6.3 kHz, looks like.

I don't think Mark needs a notch filter, rather, a good pressure mic for measuring....

[dmtp]

I am overwhelmed by the responses! Thanks for all the help, but it DOES look like my next step needs to be something better to do my FR measurements with! (I hve mostly used the RS meter for LF in the past and it seemed OK.) With fear of getting a lengthy side discussion going (if so should we move it to a new thread?), what would you suggest I consider for my next purchase? Here are a few parameters:

1. I am not a destitute college student (that was back in the 60's when I built pseudo AR-4a's in my dorm room)
2. OTH, this is strictly a hobby so I am not ready to shell out $$$$ for pro grade instruments.
3. I would consider computer based solution, but my computer is a long way from where I work on the speakers (in the basement) so getting everything together would be a lot harder than with separte equipment.

TIA for your suggestions.

[dmtp]

To clarify some of the discussions, here are 3 more SPL graphs (unfortuantely still take with the RS meter).. Again, sorry for the pdf. If anyone wants to convert to jpg and post I would be thankful.
Plain is just driver/horn
Conjugate is LR (2.0 mH, 14 ohms) with the 20 ohm parellel resistor
20 ohm is no LR, just the 20 ohm.
Previously posted curves had NEITHER the LR nor the 20 ohm conjugates
I will try to come up with better instrumentation before confusing things further!

[Zilch]

What would you suggest I consider for my next purchase?

Behringer UltraCurve Pro DEQ2496, available at Guitar Center, for example, for $250. You might be able to get them to throw in the $50 ECM8000 mic and some cables for that. You'll also need a mic stand with mini-boom, about $35 more.

So, for ~$300, you can get set up with a good stand-alone semi-pro test/measurement system. I believe those who have followed my exploits in these forums will attest it has carried my investigations a VERY long way.

I have plastered these forums with pics from its distinctive orange display. It's a 31-band equalizer as well as a 61-band RTA, with "AutoEQ," which will show what adjustments need to be made to achieve any desired response curve.

Transcribing the FR curves of driver/horn combinations into crossover optimization software, you can achieve quite good results via simulation. With a little more ($25) input protection hardware, you can measure crossover voltage drives with the same setup, too

I also like the AutoEQ because it lets me hear what the system is going to sound like before I put in the effort to tweak a crossover for it. Sometimes the answer is "Don't bother...."

[Earl K]

MarkT;

Read this thread for some ideas and opinions on test gear. Here are a couple of quickies ;

(i) Hardware ;

- One of the simplest to acquire ( and cheap to boot ) is an older Behringer DSP 8024 ( or the older DSP 8000 ) . These are two channel digital EQ units that happen to also have a decent builtin, RTA section . They "output" sine waves, white noise & pink noise at user selectable levels . They have a builtin preamp ( with phantom power for condensor mics ) on a standard 3 pin XLR connector . They eBay in the low 100s' these days .
- The necessary test mic / can be the inexpensive Behringer ECM 8000 / which is @ $ 50.00 from most musician supply stores .
- The only "problem" with these stand alone hardware units is that "sharing" with others, the FR that is seen on the screen requires taking an actual photo with a digital camera / importing the pic / then sizing the jpeg to fit into this forums size rules / and then of course, posting it . ( Having a software based system , just about always circumvents this tedious task and allows the screen captures to be manipulated in other more meaningful ways ) .

(ii) Software Based ;

- Buy an inexpensive notebook computer / then load it with TrueAudios TrueRTA / then get a decent "full duplex sound card" and use the Behringer test mic with an inexpensive preamp to feed the "duplex" soundcard . This is more money / but a better approach in the long run . The 1/6 octave or 1/12 octave versions will be the most useful .

- A slightly more expensive alternative to the somewaht "simplistic" capabilities of "TrueRTA" is ETF(s)' ETF5 Measurement Software. From what I have seen Jack Bouska accomplish with it / and at only $ 150. ( software only ) / it appears to be the bargain of the year ( though I haven't used it ) . Since it has waaaay more capabilities that TrueRTA / expect the learning curve to be longer . The company also sell a test mic/preamp combo that seems to be reasonably priced ( though I don't know any of the specs on the hardware to really comment further ) . Consider getting some private advice from Jack B. about this product .



PS ; When you get something that can capture semi "accurate" FR plots / then we can proceed further . In the meantime , build Zilches last schematic / and a notch filter if you guess that it's necessary.

[Zilch]

Here's your FR curve from the PDF. To post it here, I have used the Adobe "Snapshot Tool" to capture the portion I want to the clipboard, which I then paste into "Paint," and save as JPG for upload here.

Your results look quite good. We now know the peak at ~6-7 kHz and the HF rolloff are likely artifacts of the RS microphone. You need better measurement tools (a "bigger flashlight," as Giskard once suggested to me,) to go much further with this....

[dmtp]

I really need to thank Earl K for measuring the RS meter and preventing a lot of wasted effort on everyone's part. I am now reminded of a previous project - a hexagonal speaker designed to 'match' our bedroom furniture utilizing a 12" floor facing woofer and an MTM arrangement using Altec car speakers (I know they aren't REAL Altecs, but I got them for 10 cents on the dollar when Leachmere went out of business). Anyway, the 4" mids and 1" tweeter came with a fairly sophisticated (for car stereo) XO which I used as a starting point. Using my same RS meter, I found a broad hump @ ~6k and designed an LCR trap to tame it. Again, I used a pot for the resistor to adjust the cut. Funny thing was, in listening to music both my wife and I preferred the XO without the trap. At the time I figured it must have had something to do with room acoustics or whatever, but now I guess it was just that our ears told us the meter was lying but I didn't realize it!

[Earl K]

I really need to thank Earl K for measuring the RS meter and preventing a lot of wasted effort on everyone's part.

You're welcome / but / it actually was Zilch who did the definative measurements that show how "wonky" that SPL meter is and how it just is not up to scratch ( for this sort of design work ) .

[Zilch]

When you get something that can capture semi "accurate" FR plots / then we can proceed further . In the meantime , build Zilches last schematic / and a notch filter if you guess that it's necessary.

I'm gonna argue against the older Behringer RTAs in comparison to the DEQ2496. They're 1/3 octave resolution (31 band) and just don't provide enough detail. I have them both, and only use the DSP 8024 as a signal generator anymore.

*****

As y'all might have surmised, I built up the crossover last night, and will try to post voltage drives sometime today. It works well, with good adjustability, though there's some interaction between the MF and HF adjustments. I don't have Mark's horn to measure with it, tho, for actual FR results. The "shape" of the compensation contour is gonna matter at that level....

I am overwhelmed by the responses! Thanks for all the help....

Just havin' fun figurin' this stuff out here....

[dmtp]

You're welcome / but / it actually was Zilch who did the definative measurements that show how "wonky" that SPL meter is and how it just is not up to scratch ( for this sort of design work ) .

Sorry about that
THANKS ZILCH!

[Zilch]

MF control at -2 dB (9:30), Mid (12:00), and +2 dB (2:30)
HF control varied Min (16 Ohms), -2 dB, Mid, +2 dB, and Max (0 Ohms)

[Zilch]

HF control at Min (16 Ohms), Mid, Max (0 Ohms)
MF control varied -2 dB (9:30), Mid (12:00), and +2 dB (2:30)

[See the "Q" change?]

[Zilch]

HF control is series leg of 16-Ohm L-pad, i.e., pins 2 and 3.

It certainly makes LE85 sizzle on H91.

[Gonna try some CD horns, now.... ]

Note: This is a virtually fruitless endeavor with H91 horn/lens, in L200B, for example. The vertical dispersion narrows so drastically as to become little more than a horizontal slit of VHF:

[Zilch]

O.K., how 'bout 2344A, then?

Just happen to have a couple of those around.

+/- 2.5 dB O.K.?

I'd say there's new options for playing 16-Ohm LE85 most sweetly with this.

Expanded scale showing available HF adjustment range, bottom.